In conventional semi-conductor devices fabrication of thin, usually less than 5 microns thick, photoresist layers are spin coated on wafers sequentially during fabrication steps. The photosensitive resist is exposed through an appropriate mask, developed, then the resulting resist pattern is transferred to a metal layer by etching or plate up.
For some applications it has been found necessary to use resist layers from 5 to 50 microns in thickness, or more. However, as the thickness increases it becomes more difficult to uniformly coat wafers. Thickness variations due to features in relief on the wafer surface and a tendency for the resist to bead to a greater thickness at the edge of the wafer become significant problems.
The present invention is directed to a method of obtaining a thick, uniform photoresist or other polymer coating on a semiconductor or other wafers by means of a novel reflow technique. The present method is particularly useful for planarizing wafers which contain previously fabricated structures in high relief.